I have removed the start batteries on my bus which had 1175 CCA apiece x2, (8D's). The bus had 2 mega amp blowers (150 to 200 amp each), 1 for the A/C and 1 for the heat. Both of these systems have been removed from the bus during the conversion. My question? - can I get away with smaller start batteries - in the neighborhood of 850 CCA apiece x2? The bus has a 6v71 - Not a GMC but the theory is the same. It starts instantly and will not be used in cold climates.

I can only answer based on our actual experiences with our 73' MC-8 with 8V71. We removed the 2 8D batteries which were in a compartment on the passenger side in front of front baggage compartment. We installed 2 group 31 (approx 875-900 CCA each) in the passenger's side engine compartment approx. 2' from the starter. In almost 7 years and 30,000 miles, the only problem has been a battery that developed a bad cell. This was replaced under warranty. Like you plan to do, living and doing most of our traveling in Florida, we do not worry abour cold weathr starting. Hope this helps, Jack

According to the DD Field Service Data manual, 6V92 is recommended to have 2-205amphour batteries in parallel or 1-410 amp hour battery, for a 12V setup.The group 31's are usually rated between 95 and 125 AH, and the 8D is rated at 225 - 255 Ah.Interestingly enough, the 4D seems to fall right into the lower end of the requirements at 180 to 215 AH and is pretty easy to move around.I use 2 group 31's for my start batteries, buut, then too, I'm in South Carolina, so cold starting isn't an issue. If it became an issue, I could kick in the other 6 31's to give it a boost.By the way, your 6V71 IS a GMC engine.The information quoted here comes from a number of websites and other places, one of which you may find interesting:

I have friends that operate a small bus line in NE Oregon. It does get cold here and they told me that they had found that for them 3 Group 31s were less expensive than 2 8-ds and every bit as satisfactory.